We recently developed an app for our visitors who are mostly savvy shoppers with the intention to seek top deals on supermarket items every week. Ads are great tools to spot great deals. Most of us use our mobile phones to get important services like delivery of food. Why not use a customizable app that can list what you spend money on, easy to use, requires no personal information. It's probably one of the simplest apps you can find in the personal finance category. Currently, it's only available in iOS but if there will be demand, we can also develop for Android. As I mentioned, it is only a very simple app with only two tab screens where you can enter some items with their costs. It calculates how much you spent totally and subtracts it from your main budget.
There is no specific reason to use this app. Keep record of what cryptocurrency or stock you bought, or just enter some grocery items like milk or bread. Here is a screenshot of some screens: